We propose a biomechanical evaluation of the gait of patients with knee disease before and after total knee arthroplasty. Two condylar-type and two hinge-type knee prostheses currently used in this Institution are chosen for this investigation. The methodology developed from this study can be easily adopted to evaluate other total knee designs. The aims of the study are to provide: 1) an objective method of evaluating of the functional effect of this type of reconstructive knee surgery, 2) proper indications for the type of total knee arthroplasty based on a specific preoperative situation, and 3) important clinical parameters for reliable evaluation of knee functions where the biomechanical evaluation methods are not available. We will study 250 patients having total knee arthroplasty; preoperatively, and one, two and four years postoperatively. The basic biomechanical variables to be used are the knee joint motion in three planes, the floor-reaction forces, the fo t-floor contact patterns, and the maximum muscle torques at the knee. Important temporal and distance factors can be extracted from these variables. Properly graded clinical examinations are also included in the overall evaluation. A performance index defined based on the graded parameters will be used to assess the disease stage and functional status of the patients before and after total knee arthroplasty. The method of data analysis consist of: 1) statistical correlation, 2) harmonic spectrum analysis, and 3) phase-plot evaluation. The entire data collection, reduction and analysis procedures are automated through the use of a FM tape recorder, A-D converters and a digital computer. A feasibility study involving 95 patients and 29 age and sex-matched normals has been completed to justify this proposal.